Superheater baffling



. y 2, 1946 u E. M. POWELL ETAL 2,403,237

SUPERHEATER BAFFLING Filed Oct. '20, 1945 2 Sheets-Sheet 2 Y INVENTORS I Elna/Z Fan/ZZZ 1 I yifiid-ff fillers? Patented July 2, 1946 UNlTED STATE S PATENT OFFICE f SUPERHEATER 'BAFFLING Elno M. Powell and Ward S. Patterson, Chappaqua, N. Y., assignors to Combustion Engineering Company, Inc., New York, N. Y.

Application October 20, 1943, Serial No. 506,922

high capacity steam generating units, but this invention relates ;to severalhovel changes in the arrangement now known to the art, which will greatly increase performance, economy of manufacture, and life of equipment.

In superheaters for high pressure steam generators, it is common practice to heat the steam to 925 F. orhigher. One of the limitations has been the metal available for fabrication of the tubes in which'the steam is heated but, as progress in alloy steelsproceeds, the moreserious limitations may be in other aspects of design. For example, when heating steam to the present high temperatures, there is a definite quantity of heat absorption required'for each'pound of steam, approximately 270 B. t. u. In a convection superheater, this heat must come from the gases passing over the superheater of which there is a definite quantity, for each pound of steam, of the order of magnitude of 1.2. Therefore, each pound of gas must give-up 225 B. t. u. or over, which is. equivalent to a temperature drop of approximate- 1y 800 F.

It is well known in'the art of heat transfer that only in a counterflow superheater can the heating gases be reduced in temperature below the temperature of the heated fluid. Heat exchangers may be designed to approach the desirable counterfiow principle in varying degrees but the most efficient design will be that in which both fluids are made to how counter .to each otherin all parts of the heat exchanger. It is one vof the features of this invention to introduce improvements in the design ofa known type of superheater so that its performance will approach thatof the ideal counterflow arrangement. V

Besides economy in design, the counterfiow principle offers other advantages, which are obvious when one studies the effect on gas temperatures. For example, a common design of pendant type convection superheaterhas the tubes so arranged and interconnected that the flow of steam from tube to tube is cross current to the flow of gas.- With such a design, the'steam at nearly 2 final temperature is in tubes contacted by the coolest portion of-the gases. Obviously, with such a design, the gases leaving the superheater must be at a higher temperature than the steam leaving the superheater in order to transfer heat to the steam. Thus, if the steam is to beheated to 950 F., the exit gases must be at, preferably, not less than 1050 F. But in a convection superheater the gas temperature drop corresponding to the required heat may exceed ,800 F., thereby establishing 1850 F. or over as the gas temperature entering the super-heater. Add another 2001?. to 400 F. for the gas drop through steam'generating tubes ahead of or adjacent to the superheaterand the required gas temperature leaving the furnace becomes well over 2000 F even at the lowest load on the'unit for which the 950 F. steam has been specified. This is above the ash-fusion temperature of some coals and, obviously, any improve.- ment in design that will lower the required gas temperature isvery desirable.

"One of the most difiicult thermal desi n problems isto provide a constanty t high steam temperature over a wide range of capacity. Here the demands of the users have only been limited'by the ability of the designers. It is apparent from the foregoing illustrative calculations that if .950 F. is "requiredfrom half load to full load, the required gas temperature leaving the furnace at the low load automatically establishes sucha high temperature at higher loads as to limit the application of an ordinary convection type super.- heater.

The lowest capacity at which a superheater is expected to heat the steam to maximum temperature is called the control point. In designing for this point, it is desirable toutilize the available heat in the gases in the .most efficient manner so as to require thelleast amoun of h a ing surface in tubesarranged on the widest possible spacing andyet occupying the least possible space and requiring the lowest p ssible entering gas temperature. The least amount of heating surface under given temperature'oonditions results from using the highest product of mean temperature dif erence timeseas side heat transfer rate. Obviously, thehighest product will ,result when each of the individual factors is the highest; and, it is we'llknown in the art of heat transfer that it requires counterflow to result in the highest mean temperature difference and gas,

w transverse t the axis of tubes to give the highest heat transfer rate, other things being equ l, such as, the powerless resulting from the friction between the ases and the tube surface.

1 3 It is one of the objects of this invention to combine both these desirable features in the same ar rangement.

When operating a convection superheater at capacities above the control point, the tendency is for the steam temperature to be higher because of higher gas temperature and one method of preventing this is by decreasingthequantity of gas whichis' directed over the superheater surface. It is obvious to one skilled in the art by this control means can be obtained with the least quantity by-passed if the flow of the bypassedgases is so directed as to completely miss all, or a major portion, ofthe superheater surface; or, in any event, so as not to come in contact with superheatersurface in a manner which would result in a high rateof heat transfer. Obviously any arrangement that permits the bypassed gases to first contact considerable superheater surface in a flow path transverse to the 1 axis of the tubes is defeating the purpose of the by-pass and willseriously limit. the amount of temperature control. it is one of the objects of this invention to disclose a method ofdirecting theby-passed gases in the preferred manner, the purpos of which is to decrease the by-passed that the greatest reduction in steam temperature I damper is opened and thereby insure that the cabers not subject to the destructive action of the high-temperature gases. Such a superheater generally employs forged return bends between adjacent tubes of. the individual elements. Economy of manufacture and the principle of vertical suspension" require that the superheater tubes have their axis substantially vertical. Special quantity, the required size of the by-pass, the required size andcost of bypass' damper, and to j improve steam temperaturecontrol. 1 V Since the by-passed gas does not pass over the superheater surface, it enters the bypass at a high temperature." Furthermore, it is not always possiblewith arrangements heretofore used to cool the by-pass'ed gases to the same temperature as the main body of gases Whichhas passed over 1 the superheater. This has often resulted in two 1 streams of gases of widely different temperatures, which, without sufiicient opportunity to mix, have then been'directed over the next section of heat- .ing surface of the boiler{unit. Very frequently the water heating 'economizer follows directly 1 after the by-pass damper in the direction of gas flow, and a non-uniform gas temperatureand/or velocity entering this equipment is very undesir Iable. This inventioninclu'des a novel arrangej'ment, of a superheater-and its associated by-pass and by-pass damper, which accomplishes mixing ofthe two'gas streams in a portion of the unit where'steam is being generated and where, therefore, the stratification is of minor importance. "The arrangement has the added advantageof absorbing heat fromthe two gas streams w'hile they. are being mixed, thereby making more effective use "of the installed heating surface and increasing'the efiiciency of the unit without additional material c0st.- I r 1 In a superheater comprising an adjacent by-i pass and'its associated damper, the resistance of the two parallel flow paths must be such that the desired division of the gas stream can be accomplished. Reduction of the quantity to be by-passed is an important step in the right direction and will result from the application of gasdirecting means mentioned above; but the fact remains that whengasis directed through the by-pass, the draft loss in the parallel superheater pass is decreased and, consequently, with the damper design now in common use, the, capacity of. the by-pass is verylimited unless an unusually large by-pass and correspondingly largeand expensive damper is provided. Therefore, it is one of the features of the proposed arrangement to locate andpivot the damper in such a way asto. add resistance in the superheater pass L 7 5 heretofore used; and

means as described herein, and transverse flow of the heating gases, therefore, is necessary with this design in order to take full advantage of the desired counterflow principle. The use of a gas flow path at right. angles to the'axis of the tubes has the further advantage of high heat transfer rate even with a relatively wide spacing between tubes, and experience has shown that a .wide spacing is very desirable when burningcoals of low ash fusion temperature. But full advantage of cross flow in a long vertically-disposed pendant type superheater is not possible except by the use of the novel arrangements herein disclosed.

The present invention will be best understood upon consideration of the following detailed description of various embodiments when read in conjunctionwith the accompanying drawings in which m V Figure 1 is a sectional elevation of a steam generator with its associated superheater of the type Figures 2 to 4 are partial sectional elevations of embodiments of the improved superheater adapted to the steam generator of Fig. 1.

InFig. l the major part of the steam generating unit is located beyond the outlet at the top of the furnace l and includes a bank of tubes 5 connecting steam and water drum 2 with water drum 4. Spaced rows of tubes 6 and '1 connecting drum 4 and another steam drum 3 encase an economizer 8. The bank of boiler tubes 5 is pro- Vided at either side with baflles 9 and I0 so arranged as to cause the gases to flow from the top downwardly along the tubes of said bank. Baflle 9 has a, lower part spaced above drum 4 and extending diagonallydownward across the bank 5 to an outlet opening I I formed between thelower ends of the two baflies. Furnace I has its rear wall faced with steam generating tubes IZ'some of which are bent to .form the gases from furnace I first pass while others form a second screen l4 spaced outwardly-from the first. Preferably all of thewalls of the furnace are faced with steam generating tubes. Between the spaced screens 13 and I4 is one section I601 thesuperheater. In the ,downpass l5 inter.-

mediate the screenld and the bank .5 is located full length of pass another part of the superheater; this may be con structed' as a single group of parallel tubes inter connected for serial flow of steam throughsuc cessive tubes and extendingfor substantially the IS or it may be constructed as showniin two superimposed groups of tubes-l8 and 19 connected for seriesfiow of steam there- 1 through. I

, The gases leaving the furnace pass; over screen- I3,'section l6; of the superheaten screen 4, and thence dovmwardlyalong and parallel to the tubes a screen 13 over which of superheater sections ['9 and 18. Some "of the gases upon entering superheater section I!) may pass thereacross and enter the top of the steam generating bank '5, thence pass downwardly through said bank to outlet H. The gas fiow through said bank may be regulated by damper 20'which thus controls a by-pass around the sutake 2 l From superheater inlet header 2d steam flows downwardly and serially through superheater groups l8 and 19 to the section It as indicated by the arrows. It will be noted that the lower group [8 receives the coldest steam which then passes to the upper group 49 to meet the hotter gases before leaving that section of the superheater. In this way partial counterflow of steam and gases is established. From superheater group IS the steam then passes to superheater section It and after passing through this section is delivered to offtake header 25. In general the gases pass over superheater sections l8, I9 along or parallel to the superheater tubes. The above described arrangement is conventional.

Because the heat transfer rate with gases flowing parallel to the axis of tubes is substantially less than that when flowing across the tubes, the corresponding sections I8A, I9A of Fig. 2 of our improved superheater are arranged for a flow of the gases transverse to the axis of the superheater tubes throughout the gas-path as well as for a counterflow of the gases with respect to the steam. Figure 2 shows a partial section of a steam generator and associated superheater with superheater sections IBA and ISA through which the steam flows serially as shown by the arrows. The steam leaving the front of section ISA is conveyed by tubes 26 to tubes at the rear of section 19A. The tubes 25 extend between the two sections and across from the front to the rear thereby forming a support for a special baffle 21 mounted intermediate and separating the sections. Baffle 21 is spaced forwardly from bank baffie 9 and extends forwardly to meet a baffie 28 mounted on a row of furnace tubes which are spaced forwardly with respect to the section 18A to form a gas chamber 3!. At about the middle of section l8A a finger baffle 29 extends from the forward bank baffie 9 toward the front side of section 58A. By this arrangement the furnace gases enter the upper section ISA, flow transversely of the tubes, and in counterflow with respect to the steam flowing through the tubes thence pass downwardly and transversely over the upper portions of the tubes of section [8A in parallel fiow with respect to the steam, thence pass through gas chamber 3| and across the lower ends of the tubes of group IBA again in counterflow with respect to the steam flow.

In this arrangement of the superheater sections and baffling therefor the advantages of cross flow and counterflow are more nearly realized than heretofore and it has been found that a reduction in the total heating surface is obtained; it also permits an increase in the tube spacing, which has the advantage of reducing the gas velocities and the draft loss through the superheater as well as reducing the tendency of slag adhesion. It also permits easier removal of slag from the tubes by lancing or by soot blowers.

Above superheater section I9A is a baffle 32 which baffle extends frombaflle 9 over section 19A at :a level above the tubes of section Hi. In this way. any gas that is by-passed'through the boiler bank is forced to pass above the top of the superheater and out of contact therewith. In the .conventional arrangement ofFig. 1', the by-passed gases flow over the superheater sections l6, l9 and thence into. the by-pass so considerable superheat is generated, which tends to minimize the control of superheat through by-passing the gases because the rate of heat transfer at this point in the superheater is still relatively high; or an excessive amount of gases must be by-passed to obtain the desired control. According to the improved arrangement shown in Fig. 2, these objections are completely overcome. It is an advantage to by-pass the gases coming from the top of the furnace rather than from a point lower down in the furnace since the gases that reach the top of the furnace have given up more of their heat to the furnace tubes and are relatively cooler thereby resulting in less heat to be absorbed in the by-pass and less severe service on the baffles, dampers etc.

A further advantage of our arrangement .of Fig. 2 is that the by-pass damper is located at a lower point of the tube bank and baifle It extends downwardly across the bent parts of the tubes in the lower part of the bank so as to direct the gases downwardly into the stream from the superheater pass. The gases leaving the by-pass may be hotter than those leaving the superheater and by this arrangement intermingle with the latter below the tubes of the banks and pass over the bottom ends of the tube bank to be thoroughly mixed so as to enter the economizer at a uniform temperature. This distinguishes from the arrangement shown in Fig. 1 where because the gases flow through a shorter by-pass with less heat absorbing surface they leave hotter and are then directed upwardly into one part of the economizer which is therefore in danger of becoming overheated.

In the modification shown in Figure 3, some of the furnace tubes [2 which form the support of the bafiie 28 between the furnace and the superheater, pass between the superheater sections l8 and Ill. The path of steam fiow through the tubes as shown by the arrows is somewhat different from that in Fig. 2 with the result that the gases flow across the tubes of section [93 in counterflow with respect to the steam as in Fig. 2 thence across the upper portion of the section I BE in counterflow instead of parallel as in Fig. 2 and thence across the lower portion of section I8B in parallel flow rather than counterflow as in Fig. 2. Obviously additional superheater sections may be added in additional passes wherein the. steam flow will be generally counter to the gas flow.

In Figure 4 the gases flowinto an up-pass communicating with the furnace and the superheater sections IB, WC and I9C are all so disposed and their interconnections forfiuid flow so arranged that the gases flow transversely of the tubes and the steam flows counterflow with respect to the gases.

While several modifications of the invention have been illustrated and described in detail, it is to be understood that the invention is not limited to the precise forms or arrangements shown but comprehends other alternatives and mechanical equivalents of the apparatus herein illustrated and. falling within the scope of the appended claims.

vertically; superheater sections and extending from one wall of said gas pass to a point short of its other wall "and other balile means extending partially across 7 l Whatwe claim v V 'i1., A steam generator comprising a water-cooled furnace'defined by bottom, side and end walls and a roof with a gas outlet in one wall near the ,roof, an oiftake, and means defining a gas pass intermediate and in. communication with'said furnace outlet and boiler ofitake; a superheater as to cause flow of furnace gases across the tubes of the sections, the sections being so intercon- Q nected that fluid fiowing therethrough advances from tube to tube in major to the gas flow.

part at least counter 2. A steam generator comprising a water-cooled furnace defined by bottom, side and. end walls 1 and a roof with a gas outlet in one wall near the 1 roof; an ofitake, and spaced wall means defining j a gas 'pass intermediate and in communication with said furnace outlet and boiler olftake; a superheater includin at least two sections of 3 interconnected tubular elements arranged one above the other in said gaspass with the axes of the tubes of each section disposed substantially baffle means disposed between 'said the tubes of the lower section from said other wall to a point short of said one wall to cause ac-ross'the tubes of the lower section, the tubes 7 of the'superheater being so interconnected that fluid flows therethrough in major part counterfiow to the gas flow.

3. A steam generator comprising a water-cooled furnace defined by bottom, side and end walls and a roof with a gas outlet in one wall near the roof, an ofltake, and spaced wall means defining agas pass intermediate and in communication with said furnace outlet and boiler ofitake; a superheater including at least two sections of interconnected tubular elements arranged one above the other in said gas pass with the axes ofv the tubes of each section disposed substantially vertically; bafile means disposed between the superheater sections and extending from one wall of said gas pass to a point short of its other wall with other bafile means extending partially across the tubes of the bottom section from said other wall to a point short of said one wall to cause flow of furnace gases transversely of the tubes of the top section, and thence back and forth across the tubes of the bottom section, the tubes of the top section being so interconnected that fluid flowing therethrough advances from tube to tube in counterfiow to the gas flow and the tubes of the bottom section being so interconnected that fluid flowing therethrough from tube to tube is in parallelflow to the first'fiow of the gases thereover and in counterflow to the second flow of gases thereover.

ELNO' M. POWELL. WARD S. PATTERSON. 

